Yup, the priority to landing a/c rule is really related to taxiing a/c not entering the runway whilst something's on approach. If you're already on it, different story.
Number one priority in any sitation like this must be protection of firstly lives and a close second aeroplanes. However, based on what we've been told I fundamentally believe that the guy landing was in the wrong (assuming no ATC and no land after) and in those circumstances my secondary objective would be to make him realise that he was wrong and for him to take the proventative action - not leave it to me.
Andy